Cracking

i found the serial for the "root-me.org" user but i can validation password. i don’t understand "The validation password is the serial’s sha256 hash".

The password to validate the challenge on Root-Me.org is SHA256( serial ).
You can calculate the SHA256 on web-sites like https[:]//emn178.github.io/online-tools/sha256.html or, if you’re on linux, by using simple terminal command echo -n | sha256sum.

Than just note that you may need to try to insert hash all lowercase or all uppercase letters (I do not remember for this challenge which it is).

i sha256(serial) But it doesn’t work

I think echo -ne "(hex)\x2....,(str)serial" | sha256sum would help. This challenge to be honest involves some minute problems that we have deal with, very unneccessary but still good one though. Also it might also gave you segmentation fault then try running it different distribution to see if it resolves.